Proper care of cultivated plants requires adequate watering without overwatering. Watering is essential not only for plants grown indoors but also in many cases to supplement rain water for plants grown out of doors. If a plant however receives too much water, root rot may develop, resulting in deterioration and even the death of the plant.
Overwatering a plant with too much water at one time can easily be guarded against by providing proper drainage such as drainage holes for indoor plants. Such drainage holes allow excess water, that is water not absorbed by the soil, to seep away from the soil and root growing area of the plant. Pots with such drainage holes usually have outer containers which collect such water. Alternatively, drainage can be accomplished by placing a layer of rocks or other such material in the bottom of a pot below the soil.
A plant which is provided with proper drainage such as by the above methods or a plant being cultivated in the ground can still suffer from overwatering, not by receiving too much water at one time but by receiving water too often. It is therefore usually recommended by plant growing experts that the soil in which a plant is potted be allowed to dry thoroughly between waterings. This is the general rule of thumb for the majority of cultivated plants. There are exceptions, such as succulents which are maintained for long periods in substantially dry conditions or ferns which usually are maintained in constantly moist soil. Most plants however thrive best when they receive a thorough watering shortly after being allowed to dry.
The appropriate time interval between waterings, hereinafter referred to as the wet-dry soil cycle, varies with many factors. It will vary with the characteristics of the individual plant, the amount of water taken up by the plant's root being dependent not only upon its age but also upon the time of year. Most plants go through a period of reduced water need during the winter season. Another factor is the composition of the soil surrounding the roots. Different soils retain different amounts of water. Environmental conditions also can vary the wet-dry cycle by affecting the rate of water evaporation from the soil. These environmental conditions include such factors as temperature and humidity of the atmosphere.
Determination of the appropriate watering time of any particular plant requires either or both observation of the plant or testing the moisture content of the soil. The leaves of a plant often will droop when the plant is in need of water. Overwatering a plant however may also cause droopage of the leaves. Relying on observation of the plant itself to determine the appropriate time for watering is therefore not practical especially for those persons without extensive experience in plant cultivation.
The second method of determining a plant's water needs, testing the soil by touching the upper surface, can also lead to unreliable results because the upper layer has a tendency to dry more quickly than the lower layers due to evaporation from the surface. Testing the soil below the very upper surface is therefore the most preferred method of determining soil moisture. This type of testing by hand requires disturbing the soil repeatedly and can be unduly time consuming when the method needs to be performed on a multitude of plants.
Devices for determining the moisture content of soil below the immediate upper surface by visual observation have been known, but suffer from serious drawbacks for use in the home or in the office or in the garden. These known devices can be considered in two categories. The first category includes sophisticated moisture gauges, such as those disclosed in U.S. Pat. No. 2,801,538, U.S. Pat. No. 2,878,671, U.S. Pat. No. 3,026,718 and U.S. Pat. No. 3,045,477. These moisture gauge indicators determine soil moisture by measuring such variables as osmotic pressure. These are sensitive instruments and although apparently durable, they are too costly to be practical for casual use in the home or garden.
The second category of indicators includes devices such as that disclosed in U.S. Pat. No. 3,824,844, which are inexpensive and generally use a hydrophyllic material covered by a layer of plastic in which a hole is pierced thereby providing soil moisture access to a small portion of the hydrophyllic material. These devices, when placed partially within the soil surrounding a plant, result in the moisture content of the hydrophyllic material being proportional to the moisture content of the soil. The hydrophyllic material or a portion thereof is chemically treated so that it changes color with a change of moisture content. Visual observation of the color of this material is indicative of soil moisture. These devices, wherein a portion of the hydrophyllic material is exposed directly to organisms contained in the soil, are subject to deterioration presumably by organisms contained in the soil. These devices have also been found to react slowly to moisture changes.